With enlargement of a screen of a liquid crystal television or the like in recent years, enlargement of a backlight included in the liquid crystal television is also required. In order to cope with such a requirement for enlargement of the backlight, and further with a requirement for energy saving, price reduction and the like, a backlight including a bent-tube type fluorescent tube instead of a conventional straight-tube type fluorescent tube has been developed. Such a bent-tube type fluorescent tube is obtained by bending a substantially center part of a long straight-tube type fluorescent tube and molding the bent portion in a U-shape or in a channel-shape (squared U-shape).
In the case of the backlight using the bent-tube type fluorescent tube, temperature distribution is relatively uniform, for example, in a screen size of around less than 17 inches, and temperature difference or luminance difference in a tube axis direction of the fluorescent tube has hardly caused a problem.
However, with more enlargement of a screen of a liquid crystal television, a voltage applied to a fluorescent tube is increased by an increase in a length of the fluorescent tube, in addition to the temperature rise in an electrode portion of the fluorescent tube as a main heat generating source. In addition, due to stray capacitance between the fluorescent tube and a unit accompanied with that, leak currents are likely to be generated near a high-voltage portion. Moreover, as inverter electric power is increased accompanied with an increase in electric power of the fluorescent tube, a heat generation volume in an inverter that is arranged on the rear face of the electrode portion of the fluorescent tube is increased. As a result, the temperature difference in the tube axis direction of the fluorescent tube becomes large.
Further, due to the above-described generation of the leak currents, the fluorescent tube is lit with a total current value of rated currents and leak currents near the high-voltage portion of the fluorescent tube, which causes luminance difference generated in the tube axis direction of the fluorescent tube. In this way, with enlargement of the backlight, the temperature difference and the luminance difference in the tube axis direction of the fluorescent tube are likely to be increased, which has caused generation of luminance unevenness of the backlight.
Moreover, for example, a step of coating a phosphor of the fluorescent tube includes processing of sucking up a suspension containing the phosphor from either one of ends of a glass tube constituting the fluorescent tube. At this time, as a length of the glass tube is made longer, it becomes difficult to obtain uniform distribution of three colors of phosphors.
When the distribution of three colors of phosphors becomes uneven in the tube axis direction of the glass tube, difference is generated in the capability that the phosphors convert ultraviolet rays into visible light or rates that the phosphors absorb the visible light, thus generating color unevenness and luminance unevenness in the tube axis direction of the glass tube. Such a tendency becomes remarkable as the length in the tube axis direction of the fluorescent tube becomes longer.
As to a technology of uniforming such luminance unevenness and color unevenness of the backlight, for example, the Patent document 1 discloses an illuminating device of a transmission-type image displaying device, including a linear light source, a reflection member that surrounds a part of the light source and reflects apart of light from the light source, and a diffusion plate that irradiates direct incident light from the light source and reflected light from the reflection member, in which the light source has two or more straight-tube portions, each of which has a different length in a tube axis direction, and a plurality of light sources are alternately arranged to face each other.
Moreover, in the Patent document 2, when two cathode tubes that are bent into a U-shape to the reflecting surface side of a reflection case are arranged, the two cathode tubes are arranged in a state where, between two straight-tube portions of one cathode tube, one straight-tube portion of the other cathode tube is inserted.
Accordingly, each of the straight-tube portions of the different cathode tubes are alternately adjacent to each other, thus light from each of the cathode tubes and light reflected from the reflecting surface of the reflection case are mixed in the almost whole areas of a plurality of cathode tubes, resulting that it is possible to suppress variance in chromaticity even when cathode tubes having different chromaticity to a certain extent are used.
[Patent document 1] Japanese Laid-Open Patent Publication No. 1-276503
[Patent document 2] Japanese Laid-Open Patent Publication No. 2002-100205